But the $22.95 serving of cod brought to tables, and supplied by Boston-based North Coast Seafoods, is not always the local product restaurant executives say they pay for. DNA testing commissioned by the Globe this summer showed it to be, on that occasion, Pacific cod, which is usually much cheaper — and to many palates, not as tasty.

Union Oyster House is among several well-known restaurants that blame North Coast, one of the region’s biggest seafood suppliers, for promising one kind of fish but delivering another. The Smith & Wollensky chain, Hilltop Steak House in Saugus, Paddy’s Pub in Newton, and East Bay Grille in Plymouth all claimed they received the wrong fish — after being told by the Globe that DNA testing revealed they were serving less expensive fish than advertised. But North Coast said all the restaurants got the fish listed on delivery invoices.

The pattern of problems at businesses that buy from North Coast illustrates a glaring lack of accountability in the poorly regulated seafood trade. Handshake deals and vague invoices make it easy for mislabeling — intentional or not — to take place at the expense of consumers who end up paying more for inferior seafood.

The Globe found North Coast technically did supply the fish indicated on most invoices, but some descriptions were ambiguous. Paperwork received by Union Oyster House, for example, identified shipments as “cod,” without indicating whether it was Pacific cod or more expensive Atlantic cod.

“For what we were paying [more than $8 a pound], for what we were getting, in our eyes it was white Atlantic cod, and that’s it,” said Joe Milano, the restaurant’s co-owner. “[North Coast] has a great reputation, but something is broke here.”

Jonathan Wiggs/Globe Staff

Union Oyster House, where Linda Guson is a waitress, said it didn’t know fish sold as local was not.

“This is an unregulated industry, reputation is everything.”

Rich Polins, vice president of sales for North Coast

North Coast president Norm Stavis said the firm only promises “cod,” because the species caught off New England is in short supply. Years of overfishing and, more recently, changing ocean conditions have prompted the government to impose stringent catch limits on Atlantic cod, even though many menus across the region suggest otherwise.

“I was not aware of the restaurant’s desire to buy exclusively Atlantic cod,” Stavis said during an interview at North Coast’s headquarters in South Boston. He said what North Coast charged for the Pacific cod was fair because the special portion size and cut Union Oyster House wanted required more processing.

Money is often the motivation for fish mislabeling, but some distributors may misrepresent seafood to provide a consistent supply of fish to a business. Restaurants sometimes don’t ask many questions because diners rarely complain about the fish they are served. In the end, industry specialists say, merchants and suppliers can escape responsibility by blaming each other.

“Nobody here is taking accountability for hoisting fraud on the consumer,’’ said Tony Corbo, of Food & Water Watch, an advocacy group in Washington, D.C.

A growing seafood business

North Coast — founded in the 1950s by Stavis’s father — is one of the region’s dominant fish processing and distribution companies, with a modern facility on Drydock Avenue and another in New Bedford.

The multimillion-dollar business has about 600 employees globally, including 250 in the United States. It sells fish caught locally and internationally, as well as farmed fish, to over 1,000 restaurants, stores, and institutions nationwide.

North Coast has aggressively expanded its retail business, attracting supermarkets such as Shaw’s and Big Y with promises of sustainable seafood — species in abundant supply and caught using environmentally friendly methods. Its customer base has grown by double digits in each of the past five years, according to Rich Polins, vice president of sales.

The Globe was unable to determine how many restaurants with misrepresented fish received seafood from North Coast because some merchants declined to name their distributors. One chain with mislabeled fish supplied by North Coast took responsibility for the error and another, Dry Dock Cafe in South Boston, sold haddock obtained from North Coast that tested out to be just that.

North Coast, like other big seafood suppliers, has profited from the proliferation of chain restaurants, which often prefer to buy fish in quantity from a single vendor.

Smith & Wollensky Restaurant Group began using North Coast exclusively about a year and a half ago at its 10 US locations. The chain decided to consolidate purchasing to better control safety and quality, according to Kim Giguere-Lapine, a spokeswoman for Smith & Wollensky. So company officials were surprised to learn from the Globe’s round of DNA testing that they were not serving red snapper as listed on the menu that day, but a less-expensive spotted rose snapper.

“We wouldn’t have put red snapper if it wasn’t red snapper,” Giguere-Lapine said. “Obviously, this [testing] has shed some light, and we are digging a little deeper.”

Stavis said invoices given to Smith & Wollensky simply said “snapper” and that North Coast’s sales staff did not misrepresent the fish it was selling. But there are dozens of species of snapper, and the restaurant chain said it specifically agreed to buy red snapper. Smith & Wollensky took the fish off the menu in October because, after learning of the DNA results, it was unclear what the chain was serving and snapper had become too costly.

North Coast executives said they purchase most fish directly from boats. It arrives whole at their plants, and lot codes are assigned to catches to ensure traceability, they said.

During a tour of the headquarters, North Coast executives proudly talked at length about the company’s participation in a voluntary food safety program run by federal inspectors. They also showed off a lab North Coast maintains on site to test for freshness, pathogens, and other contaminants.

“This is an unregulated industry,’’ Polins said. “Reputation is everything.”

The right deals, at a cost

It was a pitch North Coast made to many restaurants: Dependable deliveries and consistent prices for quality New England seafood staples.

Brothers John and Karl O’Hara, who own Paddy’s Pub in Newton, went for the deal. About two years ago, they parted ways with their longtime fish monger, Captain Marden’s Seafoods in Wellesley, to buy haddock from North Coast.

But according to the Globe’s testing, Pacific cod turned up in Paddy’s fish and chips instead of the more expensive haddock advertised. When the O’Haras asked North Coast what had happened, a salesman for the wholesaler said cod was delivered that one time only. North Coast executives said that Paddy’s must have mistakenly ordered cod instead of haddock and that the invoice correctly indicated what the restaurant received.

The O’Haras acknowledged the invoice listed cod, but said they order only haddock. To back that assertion, they produced dozens of other daily invoices from North Coast showing haddock deliveries.

“Is it our dumb luck that the one day the Globe tested we got cod delivered?” Karl O’Hara said.

Kim Marden, of Captain Marden’s, said he enjoyed working with the O’Haras for more than a decade, but could not compete with North Coast’s prices.

“We couldn’t come close, and when things don’t make sense and it’s that far off, there is nothing we can do,” Marden said.

When North Coast found out more than two years ago that the Hilltop restaurant was buying frozen haddock from another company, North Coast offered to sell it fresh haddock for about 20 cents a pound less, according to Hilltop manager Donald Doward. There was one caveat, he said — instead of fillets, the restaurant would get haddock pieces left over from processing. A North Coast salesman assured Hilltop the quality would be good enough for broiled, baked, and fried entrees at the Saugus restaurant, according to Doward.

But DNA testing by the Globe found the haddock was really Pacific cod, which is almost always previously frozen. North Coast officials said Hilltop knew it was getting chowderfish — a mix of boneless cod and haddock scraps, which are similar-looking white fish. Any chef would immediately recognize it as such, the executives said, and the cheaper price should have tipped off the restaurant’s buyer.

Hilltop’s chef said he wasn’t suspicious of the fish because it arrived in relatively large white pieces, not small chunks.

“We thought it was a good deal,” Doward said.

East Bay Grille in Plymouth said North Coast has been a reliable supplier for years. But after a dish the restaurant sold as grouper — an expensive, flaky white fish found in southern waters — was found in the Globe test to be more pedestrian Pacific cod, general manager Erik Daigle was perplexed.

“It’s unlikely that a mistake was made in the kitchen, and it’s more likely a supplier issue,” he said, noting that the invoices from North Coast list grouper. “This raises a bit of a red flag. You just want to believe people are supplying you with the product they say they are giving you.”

North Coast officials said it is nearly impossible to mistake cod for grouper. They noted the Globe reported last year that East Bay Grille had served Pacific cod for other fish without telling customers. Stavis added that some restaurants serving misrepresented fish, including East Bay Grille, receive seafood from multiple vendors, so it’s unclear which one delivered the Pacific cod to Plymouth.

But North Coast was also identified as the source of mislabeled fish at Not Your Average Joe’s in Westborough as part of the Globe’s 2011 investigation.

The restaurant chain said North Coast supplied southern bluefin tuna, considered a critically endangered species by an international conservation group, instead of the yellowfin promised on invoices, a contract, and on the restaurant’s menu.

North Coast officials last year denied selling bluefin tuna to Not Your Average Joe’s. But this year, Stavis conceded a bluefin may have been caught accidentally as it swam alongside yellowfin tuna and been mistakenly processed.

A fixable problem?

Industry specialists say there are ways to make the internal workings of the fish trade less mysterious for restaurant operators and consumers. Some suggest adopting a system like the one used by the European Union, the world’s biggest importer of fish. It maintains a list of approved countries that meet EU standards of seafood inspections and testing. The United States has such a system for meat, but not for the 127 countries that ship seafood here.

Others say regulators could require suppliers and restaurants to identify where fish are from. Supermarkets must label the country of origin, but restaurants are not required to.

“People want to know this,’’ said James Wright, senior editor of SeaFood Business, a trade magazine.

In addition to better oversight, industry specialists say chefs and suppliers must be vigilant to prevent one fish from being substituted for another.

For example, Union Oyster House relied on an oral agreement for its cod. Other restaurants assumed they received the correct product simply because they asked for it. Errors could be reduced if written purchase agreements listed the exact species and restaurant operators audited invoices to ensure there were no discrepancies.

“If you manage the mistakes, they should diminish to a reasonable level, and if they don’t, you’ve got a bad partner and it’s time to get a new partner,” said Dennis Lombardi, a food service industry consultant at WD Partners in Ohio.

North Coast executives said they have no obligation to make sure restaurant menus are accurate. But they conceded menu mistakes and deception ultimately hurt the company.

“We’re only as good as our customers,” Polins said. “Truth in menu is very important.”

Union Oyster House co-owner Milano said the restaurant plans to be more careful about ensuring the Boston cod dish it serves is what customers order. And even though locally-caught cod is in short supply overall, he now believes there’s enough for his restaurant.

“North Coast assures me now what we are getting is Atlantic cod,” Milano said.